I don’t think it’s as simple as that. DN was primarily a text collection for Non-Buddhists, but it doesn’t mean no other texts could be added to it earlier or later for any other reason. Take the two last Suttas in DN: they are just long lists of Dhamma topics. DN 15, due to its highly speculative doctrinal contents could not be possibly directed at the non-Buddhist audience. If I would show it to my girlfriend, she wouldn’t really understand much, whereas she could easily connect to much of the Mahaparinibbana Sutta.
How, why and when DN 15 was compiled and how it can be compared to other Suttas is a difficult question that I am not really ready to answer without first consulting any papers on its Chinese Agama counterparts. Unfortunately, I have no information on such papers except this one by Bucknell 
Anyway, I should warn you I am very sceptic about how unconvincing Bucknell’s arguments are: the shorter version of the Dependent Origination fits the teachings by Venerable Sariputta so nicely it can hardly be a coincidence. Besides, as I said multiple times elsewhere, in my opinion it is crucial that the Buddha says ‘to this extent designations etc. go’, as if specifying the difference between the shorter and longer DO formulae. In other words, I think, it may well be possible the first two nidanas cannot be satisfyingly described linguistically. And don’t forget there is one more deviating DO formula, in the Udana, a book that is scarcely Brahminical.
Just as I mentioned above, I don’t think specifically DN 15 is ‘Brahmanism in disguise’ because it fits so perfectly with Ven. Sariputta’s simile of two bundles supporting each other and otherwise doesn’t contradict the Buddhadhamma in any way, neither does it disavow the longer version. Honestly, I know of no Sutta that is Brahminical (even though my competence is limited), but I know of certain interpretations of Suttas that are kinda itty-bitty Brahminical, and the Original Mind or vinnana-nibbana theories kinda smack of, you know, the Upanishads 